In order to form a minute pattern using a resist, there is generally used a lithography technique in which a solution containing the resist is applied onto a substrate and then dried to form a resist film (photosensitive film) thereon, the resist film is exposed to activated radiation to form a latent image, and the latent image is then developed to form a negative or positive image.
When a semiconductor element is fabricated by minute processing making use of a resist, a silicon wafer is used as a substrate to form an image (resist pattern) thereon by the above-described lithography technique. After the substrate is then etched by using the resist pattern as a protective film, the remaining resist film is removed.
With the advancement of high integration, high density assembly, miniaturization and speed up of semiconductors toward IC, LSI and further VLSI, demands for minute processing of semiconductor elements are increased, and there is a demand for development of a technique for forming a minute pattern having a line width of 0.5 .mu.m or less at present. It is however extremely difficult to form such a minute pattern by any of the conventional lithography techniques making use of near ultraviolet radiation or visible radiation. Its yield is also lowered to a significant extent.
Therefore, in order to enhance the resolution of exposure, there are studied other lithography techniques making use of far ultraviolet radiation (short wavelength ultraviolet radiation), KrF excimer laser beam or the like, which is shorter in wavelength than near ultraviolet radiation, in place of the conventional lithography techniques making good use of near ultraviolet radiation having a wavelength of 350-450 nm.
When the short wavelength ultraviolet radiation or KrF excimer beam is used, however, a problem that the conventional resist compositions comprising an o-quinonediazide compound as a photosensitive agent cannot be adapted to the use of radiation short in wavelength for exposure with the object of improvement in resolution because the wavelength for exposure is fixed has arisen.
In recent years, there has therefore been developed a resist composition for minute processing, which is composed of a three component system of (1) a base polymer (resin binder), (2) a compound (photo-induced acid generator) which generates an acid upon exposure to light and (3) a substance (acid-sensitive substance) which reacts in the presence of an acid as a catalyst, which has been generated from the photo-induced acid generator, to change the solubility of the base polymer in a developing solution (Japanese Patent Application Laid-Open No. 107160/1991). Resist compositions composed of a two component system of 1 a photo-induced acid generator and 2 a compound or resin binder which is decomposed in the presence of an acid as a catalyst, which has been generated from the photo-induced acid generator, to change its solubility in a developing solution have also be proposed. Specifically, for example, acetal compounds (Japanese Patent Application Laid-Open No. 89003/1975), enol ether compounds (Japanese Patent Application Laid-Open No. 12995/1980), polymers having an orthoester group in their main chains (Japanese Patent Application Laid-Open No. 17345/1981) and polymers having a tert-butyl ester group of a carboxylic acid or a tert-butyl carbonate group of phenol (Japanese Patent Publication No. 27660/1990) have been proposed as the compound or resin binder used in combination with the photo-induced acid generator. However, these conventional resist compositions are still insufficient in performance from the viewpoints of sensitivity, storage stability, stability with time, resolution, pattern form and the like. Therefore, further improvements are required of these compositions.
In more recent years, in order to attain higher sensitivity and heat stability of a resist, there has been proposed resist compositions in which a t-butoxycarbonyl group, t-amyloxycarbonyl group, t-butyl group, t-hexyl group, allyl group or 2-cyclohexyl group is bonded, as a protecting group decomposed by an acid or base, to an alkali-soluble resin (Japanese Patent Application Laid-Open No. 158363/1992). The alkali-soluble resins into which these protecting groups have been introduced are protected from dissolution in an alkaline developing solution. However, these protecting groups are decomposed by an acid derived from, for example, a photo-induced acid generator, and so the protected alkali-soluble resins turn soluble in the alkaline developing solution. Besides, a resist composition making use of a compound having a tert-amyl ester structure in its molecule as an acid-sensitive substance has been proposed (Japanese Patent Application Laid-Open No. 269754/1992). Although improvements in resist properties are recognized by such various investigations, further improvements are requested in view of higher properties, in particular, higher resolution required of resists.
In addition, a resist pattern formed with any of the conventional resist compositions has involved a problem that it undergoes deterioration of form (heat deformation) by heat generated in an etching process, resulting in a failure to exactly transfer the form of the resist pattern to a substrate.
In order to solve this problem, it has been proposed to introduce, as a protecting group to be decomposed by an acid or base, a group capable of forming a secondary carbonium.ion intermediate having a proton usable in a reaction upon decomposition into an alkali-soluble resin, thereby improving the resistance to thermal decomposition of the resulting resist pattern (Japanese Patent Publication No. 19139/1993). The resist pattern formed with the resist composition described in this document is recognized to have improved resistance to thermal decomposition compared with a resist composition containing a polymer having a t-butyl group as a substituent group of an acid-labile group (protecting group decomposed by an acid). However, its heat deformation property is not recognized to be improved, and the sensitivity required of a resist film is insufficient.
As the latest example, a resist composition comprising a copolymer of hydroxystyrene and a (meth)acrylate has been proposed (Japanese Patent Application Laid-Open No. 266112/1994). As specific examples of an acid-labile group bonded to the copolymer, are mentioned t-butyl and .alpha.-methylbenzyl groups. Improvement in resolution is recognized by using this resist composition. However, the improvement is not yet satisfied, and so there is a demand for still more improvement of resolution.
As described above, there has not been obtained under the circumstances any resist composition which is hard to be deformed by heat in a technique of forming a resist pattern using short wavelength ultraviolet radiation or KrF excimer laser beam as activated radiation, and is also excellent in balance among resist properties such as sensitivity and resolution.